To flower in spring, fall bulbs require exposure to cold temperatures. This chilling period triggers them to break dormancy when temperatures warm in spring. It's all about planting early and warm enough for the bulbs to begin developing, but late enough for them to stop developing shortly after to start soaking in the cold.
Cooler soil temperatures are a primary indicator for proper planting time. Ideally, soil temperature needs to be below 60F; a temperature of 55F degrees is perfect. The problem for the average gardener is that few gardens come with soil thermometers. As a guideline, soil temperatures usually cool to this target after a few weeks of nighttime temperatures regularly between 40 and 50.
There are also natural indicators that some gardeners use:
Plant just after fall foliage peaks
Plant when you no longer hear crickets at night
Plant when you see flocks of birds heading south
Plant when you regularly need a jacket to work in your garden
Plant the week after you smell your neighbor's fireplace for the first time
Plant when you have to turn your car heater on in the morning
Plant when your dog moves from the shade to the sun
Plant when your grapes are ripe
Plant on Columbus Day, Halloween, Veteran's Day, or Thanksgiving (depending on your zone)
Plant garlic on the first day of school
Plant after you blow out your lawn sprinklers
These suggestions are very unscientific and rely on local conditions that may not match your climate. But if you find a natural indicator that works for you, use it.
Some gardening experts recommend planting bulbs six weeks before a hard, ground-freezing frost. That's a little too hard to predict for many regions. If unseasonal warm weather lingers, bulbs may end up waiting in the ground for the cold to arrive for two months or more. That's much too early.
Planting within a few days of your average first frost date is a good guideline. That's what I typically use. The average first frost date means that historically half of first frosts occur before that date and half occur after. For me, that's the first week of October. By that time cool nights have cooled the soil but there are still enough warm days ahead to keep the soil warm enough for root development.
A little too late is usually better than a little too early. There is a four to six-week target window for planting bulbs, but as long as you can still work in the soil it's probably not too late. Crocus, scilla, and snow drops do better planted earlier. Daffodils, hyacinth, and tulips can handle late planting. In fact, tulips can be planted in frozen ground and will probably still do fine.
If you miss the ideal planting time and still have bulbs you purchased, plant them anyway. Most bulbs will dry out and be worthless if left to sit in their bag over the winter. They stand a chance of growing while in the ground. Plant them and hope for the best; you may be surprised by the results.
If you’re still wondering when to plant your bulbs take a look around your garden. When you walk outside at night are you cold without a jacket? Have your pepper and tomato plants withered from frost? Are your raspberries fruiting? Are mum flowers drying on the plant? These are all signs that the season is changing and winter is coming. Don't delay too long. It may be the perfect time to put those bulbs in the ground.